Can somebody provide some clarification: I am calculating the stress in a plate with the following dimensions: 41" long by 30" wide and the plate is 3/8" thick. The plate is simply supported along the 41" length sides, with the short lengths free. I have calculated the stress using beam theory with a point load of 18815N at the centre of the plate as 227N/mm2. However, I have a Roark book and I'm looking at the tables for 'formulas for flat plates with straight boundaries and constant thickness'. I can't find the exact formula I want but have used a formula for a rectangular plate simply supported at all edges with a uniform load over a central rectangular area (I've assumed this area to be as small as possible to try and best represent a point load) My question is, when do I use plate theory and when do I use beam theory? And is there a plate theory formula for 2 simply supported sides and 2 free sides?